Now those things are clarified lets analyze the system again:

1) Before it starts the mass is stationary and the pipe is stationary:
pMass = 0
pPipe = 0
pTotal = 0 + 0 = 0

2) After the first acceleration the mass is moving and the pipe is moving in the opposite direction:
pMass = -1
pPipe = 1
pTotal = -1 + 1 = 0

3) After the first turn the mass has the same speed but opposite direction.
pMass = 1
pPipe = ?
pTotal = 1 + ? = ?